EP2999950B1 - Sniffing leak detector having a nanoporous membrane - Google Patents

Description

The invention relates to a sniffer leak detector for aspirating a gas to be analyzed.

A sniffer leak detector is used for gas analysis and is equipped with a sniffer probe for aspirating the gas to be analyzed. The gas analysis is typically carried out with a mass spectrometer in a high vacuum. In mass spectrometric gas analysis, air at atmospheric pressure (ambient air) is typically drawn in the vicinity of a suspected leak in a test specimen. The test specimen is filled with a test gas such. As hydrogen or helium filled. The test gas pressure inside the test specimen is greater than the atmospheric pressure of the environment, so that the test gas escapes through a leak from the test specimen and into the air in the area of the test specimen Environment of the specimen arrives. The air sucked in with the sniffer probe is admitted into the high vacuum in the main or partial flow, where it is measured by means of a mass spectrometer the partial pressure of the test gas (hydrogen or helium).

A critical measure of the quality of the measurement is the detection limit of the sniffer leak detector for the test gas. The detection limit is the minimum detectable concentration of the test gas in the intake air. The lower the detection limit, the more sensitive the measuring system is and with even greater accuracy the test gas content can be determined.

It is known to arrange in the gas inlet in the high vacuum of the mass spectrometer, a gas-permeable membrane, which is flowed through by a part of the sucked gas. The known membranes are sintered ceramic disks which are intended to favor the relatively light test gas helium or hydrogen and to pass less of the heavier gas components. For a mass spectrometric gas analysis with direct gas inlet into the high vacuum of the mass spectrometer (total pressure <10 ^-4 mbar), the known sintered ceramic discs are suitable. With gas inlet into the pre-vacuum of the high vacuum pump, as in the case of a countercurrent leak detector, the conductance is insufficient to produce the necessary gas flow, which is about 100 times higher.

US 2008/202212 A1 describes a leak detection method including a leak detector including a test gas inlet, a vacuum pump connected to the test gas inlet, and a test gas sensor connected by a path to the test gas inlet in which a membrane permeable to the test gas is located.

DE 4326267 A1 discloses, as a post-published prior art, a leak detector with a selectively acting with respect to the passage of gases membrane.

The object of the invention is to improve the detection limit of a sniffer leak detector for mass spectrometric gas analysis by providing a sufficiently large but nevertheless molecular conductance, which preferably admits hydrogen to heavier gases of the air.

The sniffer corner finder according to the invention is defined by the features of claim 1.

In the sniffer leak detector according to the invention, the gas inlet to the mass spectrometer via a porous membrane flowed through by the aspirated gas whose pore diameter is less than or equal to the free path of air at atmospheric pressure and at room temperature. The atmospheric pressure is considered to be in the range of about 950 hPa to 1050 hPa. As room temperature, a temperature in the range of about 15 ° C to 25 ° C is considered. According to the invention, it has been recognized that pores having a diameter which corresponds at most to the free path of air at atmospheric pressure and room temperature produce a molecular gas flow even at relatively high pressure prevailing in front of the inlet membrane of a sniffer leak detector. Here, the conductance for the light test gases hydrogen or helium is particularly high, while the conductance of the heavier, undesirable in the analysis of gases is low. Thereby, a molecular gas flow containing the test gas is generated in the vacuum, which is not viscous, but in which the different molecules move independently of each other and at different speeds. The light gases, which include the test gases hydrogen and helium, are moving very fast, which means that their proportion is higher in a high vacuum than in the intake gas stream and thus the detection limit is improved. Although a certain accumulation was achieved with the previous sintered membrane technique, the gas flow admitted is so small that the detection limit is even worse than with direct inlet (for example via a diaphragm).

The invention is thus based on the idea of making the pore openings as small as possible and preferably with the same diameter as possible. It is particularly advantageous to provide as many pores as possible in order to pass a comparatively large amount of gas despite the small pore size.

Similar membranes are known from another field of technology - namely the ultrafiltration of macromolecules in liquids - and serve there not to improve the detection limit of a sniffer leak detector but a defined filtering of macromolecules with high accuracy.

For example, the pore diameter may be less than or equal to 20 nanometers (nm). The diameter of each pore should be at most about 50% and preferably at most about 20% different from the mean diameter of all pores, so that the pores of similar size as possible, so as not to pass unwanted, heavy gases even with large pressure differences.

Nevertheless, in order to allow a sufficiently large proportion of gas to pass through, the area fraction of all the pores should be at least about 20% and preferably at least 40% of the total membrane surface area. The area fraction of all pores can be in a range between 25% and 50% of the membrane surface.

The pore density should be as large as possible. Preferably, the membrane should have at least 20 and preferably at least 25 pores per square micrometer (μm ² ) of its surface. The wall thickness between adjacent pores, ie the smallest distance between the edges of adjacent pores, should be as low as possible and less than 100 nm and preferably less than 80 nm.

The slice thickness of the membrane should be less than 100 .mu.m and preferably less than 50 .mu.m and possibly only a few tens of microns or less in order to minimize the length of the pores.

womit sich bei ca. 1000 mbar eine mittlere freie Weglänge von $$\frac{\mathrm{6,65}\cdot {10}^{-5}m\cdot \mathit{mbar}}{1000\mathit{mbar}}=\mathrm{66,5}\mathit{nm}$$

ergibt.The quotient of the mean diameter of all pores and the mean free path of the sucked Gases (air) at atmospheric pressure and room temperature may be greater than 0.5. This quotient is called the Knudsen number. For the mean free path I and the pressure p of the intake air: $$\tilde{\mathrm{I}}\cdot \mathrm{p}=6.65\cdot {10}^{-5}\mathrm{m}\cdot \mathrm{mbar}\left(\mathrm{at}273.15\mathrm{K}\right)$$

which at about 1000 mbar a mean free path of $$\frac{6.65\cdot {10}^{-5}m\cdot \mathit{mbar}}{1000\mathit{mbar}}=66.5\mathit{nm}$$

results.

With the sniffer leak detector according to the invention, it is possible to generate the maximum high vacuum pressure of 10 ^-4 mbar with the countercurrent gas introduced via the backing vacuum, which effects the best possible detection limit in the mass spectrometric gas analysis.

The features of the invention are particularly simple and reliable to implement in a nanoporous membrane of alumina.

Figur 1

eine schematische Darstellung des Schnüffellecksuchers und

Figur 2

einen mikroskopischen Ausschnitt einer Draufsicht auf die Membrane.

In the following an embodiment of the invention will be explained in more detail with reference to FIGS. Show it:

FIG. 1

a schematic representation of the Schnüffellecksuchers and

FIG. 2

a microscopic section of a plan view of the membrane.

In Fig. 1 the sniffer leak detector 10 according to the invention is shown, which consists of a sniffer probe 12, a feed pump 13, a mass spectrometer 14 and a vacuum pump 15, 16. The sniffer probe 12 is provided with a Feed pump 13 for the suction of gas through the sniffer probe 12 connected gas-conducting. The gas drawn in by the feed pump 13 through the sniffer probe 12 is supplied to the gas inlet 17 of a turbomolecular pump 15. The turbomolecular pump 15 forms together with an associated backing pump 16, the vacuum pump 15, 16 for the mass spectrometer 14. The gas inlet 17 know a gas-permeable, porous membrane 18, through which the gas is sucked into the turbomolecular pump 15. For this purpose, the turbomolecular pump 15 is gas-conducting connected to the mass spectrometer 14 for its evacuation. Valves or pressure gauges are not needed.

The mass spectrometric sniffer leak detector 10 is a countercurrent leak detector for light gases. The gas is admitted in the pre-vacuum of the vacuum pump 15, 16 and not in the high vacuum of the mass spectrometer 14. In this case, the slight proportion of the sucked gas preferably diffuses into the mass spectrometer 14. As a result, a large amount of gas can be sucked in to a particularly high sensitivity while enriching the light gas across the membrane 18.

A microscopic section of a plan view of the surface of the membrane 18 is shown in FIG Fig. 2 shown. The membrane 18 has a plurality of pores 20 which are randomly distributed over the surface of the membrane 18. Each pore 20 passes completely through the membrane 18. The membrane is a disc having a thickness of about 30 microns, so that the length of each pore 20 is about 30 microns. The length of each pore 20 is therefore equal to the thickness of the membrane 18th

Fig. 2 shows that the membrane 18 has about 26 pores per μm ^{2 of} its surface. The mean minimum distance d of adjacent pores 20 (midpoint - midpoint) is 100 nm. With the mean minimum distance, the mean of all the smallest is measured from midpoint to midpoint of the pores Mean distances directly adjacent pores meant. The mean diameter D of all pores 20 is 20 nm and, in an alternative embodiment, may also be less than 20 nm.

The area fraction of all the pores 20 on the surface of the membrane 18 is 50%, so that a total of half of the membrane surface is gas-permeable.

The invention is thus based on the idea that as a gas inlet not a diaphragm with only one opening, but rather a gas-permeable, porous membrane is used, meet the individual holes at the prevailing pressure at the gas inlet Knudsen condition for molecular flow. The hole density is chosen so high that, despite the small pore size such a gas amount is transmitted that the high vacuum pressure of 10 ^-4 mbar can be achieved. Here, the physical principle is used that at molecular gas flow, the gas components of a gas stream move independently (molecular) and each have their own conductance. Molecular conductances are inversely proportional to the root of the molecular weight of the particular gas. Hydrogen therefore has a much better conductance through a given orifice than nitrogen and oxygen, and as all other constituents of air.

Claims (9)

1. A sniffer leak detector (10) for sucking in and analyzing gas, comprising a sniffer probe (12), a gas-conveying pump (13) connected to the sniffer probe (12) for sucking in the gas through the sniffer probe (12) with atmospheric pressure, a vacuum pump (15, 16) having a gas inlet (17), the gas inlet having a gas-permeable membrane (18), and comprising a mass spectrometer (14) connected to the vacuum pump (15, 16) for analyzing the sucked-in gas in a high vacuum, wherein the gas flow sucked in by the gas-conveying pump through the sniffer probe is conducted along the membrane (18) such that the membrane (18) allows part of the gas to flow into the prevacuum of the vacuum pump (15, 16) for the mass-spectrometric analysis of the gas in the high vacuum,
   characterized in that
   the membrane (18) is a porous membrane having gas-permeable pores (20), wherein each pore (20) penetrates the membrane (18) completely and the diameters (D) of the pores (20) are less than or equal to the free path length (I) of air at atmospheric pressure and room temperature, and
   that the surface ratio of all pores is at least about 20% of the total membrane surface area.
2. The sniffer leak detector (10) of claim 1, characterized in that the mass-spectrometric sniffer leak detector (10) is a counterflow leak detector.
3. The sniffer leak detector of claim 1 or 2, characterized in that the pore diameters are each less than or equal to 20 nm.
4. The sniffer leak detector (10) of one of the preceding claims, characterized in that the diameter of each pore (20) differs from the mean diameter of all pores (20) of the membrane (18) by a maximum of 50% and preferably by less than 20%.
5. The sniffer leak detector (10) of one of the preceding claims, characterized in that the total surface ratio of all pore openings is at least 25% and preferably at least 40% of the total surface area of the membrane (18).
6. The sniffer leak detector (10) of one of the preceding claims, characterized in that the membrane (18) is a disc with a thickness of less than 100 µm and preferably less than 50 µm.
7. The sniffer leak detector (10) of one of the preceding claims, characterized in that the membrane (18) is a nanoporous disc of aluminum oxide.
8. The sniffer leak detector (10) of one of the preceding claims, characterized in that the smallest distance between adjacent pores (20) is less than 100 nm and preferably less than 80 nm.
9. The sniffer leak detector (10) of one of the preceding claims, characterized in that the membrane (18) has at least 20 and preferably at least 25 pores (20) per µm² of its surface area.

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